The most likely time and place of introduction of BTV8 into Belgian ruminants

Abstract

Background: In northern Europe, bluetongue (BT) caused by the BT virus (BTV), serotype 8, was first notified in August 2006 and numerous ruminant herds were affected in 2007 and 2008. However, the origin and the time and place of the original introduction have not yet been determined.

Methods and principal findings: Four retrospective epidemiological surveys have been performed to enable determination of the initial spatiotemporal occurrence of this emerging disease in southern Belgium: investigations of the first recorded outbreaks near to the disease epicenter; a large anonymous, random postal survey of cattle herds and sheep flocks; a random historical milk tank survey of samples tested with an indirect ELISA and a follow-up survey of non-specific health indicators. The original introduction of BTV into the region probably occurred during spring 2006 near to the National Park of Hautes Fagnes and Eifel when Culicoides become active.

Conclusions/significance: The determination of the most likely time and place of introduction of BTV8 into a country is of paramount importance to enhance awareness and understanding and, to improve modeling of vector-borne emerging infectious diseases.

Figure 1. Study area and random milk tank survey.

Southern Belgium with its 5 provinces and location of 206 randomly selected dairy herds tested for BTV antibodies on milk tank samples (black cross, the 156 dairy herds tested negative during the period between 12^th January 2006 and 14^th March 2006; black triangle, the 50 dairy herds tested negative during the previous period but tested positive during the period between 6^th December 2007 and 3^rd February 2008).

Figure 2. Time of first clinical BT appearance ([A], [C]) and the duration ([B], [D]) between this time and the first outbreak notification to the Authorities (87 first outbreaks in province of Liège).

Date: day/month/year; [A] and [B]: 57 cattle outbreaks; [C] and [D]: 30 sheep outbreaks; For [B] and [D], the vertical dotted line corresponds to a duration of 7 days.

Figure 3. Spatial distribution of Cq values observed in cattle herds in the eastern part of the province of Liège during the first 2 weeks after the appearance of bluetongue.

[A] and [B]: Areas close to and remote from the National Park of Hautes Fagnes and Eifel, respectively. The two areas were delimited according to a distance of 30 km from the centroid of the National Haute Fagnes – Eifel. BE = located in Belgium; DE = located in Germany.

Figure 4. Starting month of clinical appearance and recovery ([A], [C]), and morbidity, mortality and case fatality rates ([B], [D]) for cattle and sheep outbreaks in southern Belgium (random surveys).

[A] and [B]: 144 cattle outbreaks confirmed by RTqPCR and/or competitive ELISA; [C] and [D]: 23 sheep outbreaks confirmed by RTqPCR and/or competitive ELISA; Later: if recovery was observed after December 2006; Never: if recovery was not observed before the end of your study (maybe those cows are recovered by now); For [A] and [C], only 2 cattle breeders and 1 sheep breeder observed BT clinical appearance before May 2006 and June 2006, respectively but there is not further information regarding the precise time point so these observations are not cited in the figure; For [B] and [D], the vertical dotted line corresponds to a rate of 5%.

Figure 5. Temporal [A] and spatial [B] distributions of 2 disease non specific indicators at the dairy herd level.

DMY: Drop in milk yield (mean by animal and monthly milk control expressed in kilograms); MCS: mean cellular score; ▪: DMY or increase in MCS; nd: Not determined; 1 to 12: Months for 2006 or 2007, respectively; For [B], X axis and Y axis are geographical Lambert coordinates; Li: Province of Liège; Br: Province of Brabant Wallon; Na: Province of Namur; Lu: Province of Luxembourg; Ha: Province of Hainaut; BE: Belgium; DE: Germany.